The application described herein relates generally to gas turbine engines, and more specifically to methods and apparatus for operating gas turbine engines.
Gas turbine engines typically include an inlet, a fan, low and high pressure compressors, a combustor, and at least one turbine. The compressors compress air which is channeled to the combustor where it is mixed with fuel. The mixture is then ignited for generating hot combustion gases. The combustion gases are channeled to the turbine(s) which extracts energy from the combustion gases for powering the compressor(s), as well as producing useful work to propel an aircraft in flight or to power a load, such as an electrical generator.
During engine operation, significant heat is produced which raises the temperature of engine systems to unacceptable levels. Various lubrication systems are utilized to facilitate lubricating components within the gas turbine engine. The lubrication systems are configured to channel lubrication fluid to various bearing assemblies within the gas turbine engine and to at least one external generator. During operation, heat is transmitted to the lubrication fluid from heat generated by sliding and rolling friction by components like bearings and seals within the engine and generator. To facilitate reducing the operational temperature of the lubrication fluid, at least one known gas turbine engine utilizes separate heat exchangers, one for the engine lubricating fluid and one for the generator lubricating fluid, to cool the fluid circulating within.
Conventionally, both heat exchangers were mounted to the inside of the shroud which encases the fan assembly. However, as the heat loads of modern engines and generators increase, heat exchangers large enough to sufficiently cool the fluids no longer fit in space allotted in the shroud. Therefore, the heat exchangers are separated such that one may be located in the shroud while the other is mounted to the engine core.
Furthermore, when the engine is non-operational or is operating in circumstances where the engine is subject to subzero temperatures, cooling of the engine lubricating fluid is not required, and a bypass valve is engaged to prevent engine lubricating fluid from flowing through the heat exchanger. Because the hot engine fluid is not flowing through the exchanger, the exchanger decreases in temperature such that any engine fluid remaining within increases in viscosity and begins to congeal. When the bypass valve is disengaged to allow flow of engine lubricating fluid through the exchanger, the low temperature of the exchanger causes the flow of engine fluid to congeal before it can warm the exchanger to allow the engine fluid to flow.
Accordingly, there exists a need for a heat exchanger that combines multiple fluid systems and prevents the congealing of fluid when the engine is subjected to subzero temperatures.